How to Match an Existing Gable Roof & The Truss Design

Adding on to a house or structure is a great way to get more room without building an entire new house! There are several important things to know about matching existing roofs and how to do it depending on what style of roof you have, the roof truss design, and how you want to add on.

Categories of House Additions

There are two main categories that additions fall under most of the time: T-shaped & continued.

Either the addition is added to form a T-shape to the house with the ridge running perpendicular to the main house roof, or the addition is continued on the end of the house making it longer with the ridge running the same way as the existing house.

Gable End Addition

Here we will focus on adding length to the end of a house roof. Now once again, there are several ways to add on.

The addition can be wider than the existing house, narrower, or the exact same width. If you choose to go wider or narrower, there won’t be the challenge of matching up exactly to the old roof and the project will be much easier. With the roof pitch the same, it doesn’t matter if the new addition is slightly off in height or lined up front-to-back because you’re going over or under the old roof.

But it is important that the roof pitch is exactly the same as the existing roof because you will see it from one roof to the other if it isn’t.

If you choose to go with the same width, it will be more difficult to execute without a noticeable difference or bump in the roof between the old and the new.

Let’s dive into a few more details regarding the roof so we can focus on matching each roof and having a seamless project with beautiful results.

First up, an example…

26-Foot Addition Example

For the first example, let’s say you have a simple 30’x60’ rectangle shaped house with gable style ends and a 2-foot overhang like this:

And you have decided that you want to add on a 26’ garage on the end of the house like this:

And the finished roof plan when you’re done with matching ridge, overhang and soffit lines should look like this:

But wait…

Do you know how your old roof was built?

Was it built with roof trusses or stick frame? Now is the time to check before you start pouring your slab and building your new walls. If you don’t check, you will likely build your new walls the same height as your old walls but there are reasons not to do so.

What are they?

Let’s explore the differences between stick framing and roof trusses, and how they affect your slab and walls:

The Two Different Roof Framing Styles: Stick Frame & Trusses

If your house was built using roof trusses, then it will look like this above the outside wall of your house:

A stick framed roof will have a rafter member looking something like this:

You’ll notice the difference in how the boards are cut to sit on the wall and create overhang.

Wood roof trusses bring the bottom chord of the truss all the way to the outside of the wall, include a small vertical cut (butt cut), and angles back against the top chord of the truss. The top chord smoothly extends out past the wall to create the overhang.

(If the roof truss has a 2×4 top chord, the heel is shorter than if it has a 2×6 top chord as a general rule. The heel height is adjustable in height by changing how tall the butt cut on the bottom chord is.)

Since the rafter has no bottom chord to sit on, it must have a birds mouth cut into the bottom side of the rafter so it has a flat place to sit down on the wall. Now the rafter also has a vertical heel cut that instead of sitting above the wall, actually sits down beside the wall. Depending on what size of rafter is being used, the corner of the birds mouth cut and the top edge of the rafter can get rather thin.

Matching the Two Different Roof Framing Styles

This brings us to the point that putting a rafter beside a wood truss or a truss beside a rafter on the same height of wall may not always work. If the stick framed rafter is a 2×6, it typically has just enough heel height after the birds mouth cut that a regular 2×4 truss can sit beside it and match height.

Any rafter bigger than a 2×6 shouldn’t be a problem unless your roof is pitched fairly steep. A 2×4 rafter just doesn’t leave enough height after the birds mouth to match on any roof pitch.

So now the first step to adding on is to determine how your current roof is built and what the heel height is.

Typically, the best way to find out the heel height is through a vent in the soffit or even pulling a piece of soffit down a bit.

Another way would be to pull off some siding at the top of the wall, on the gable end, right where you are going to be starting the addition. Once you can reach the heel, carefully measure the height of it vertically at the outside corner of the wall to within 1/8 inch.

Also determine how the roof was built while you’re looking at it – roof trusses or stick framed?

If the roof is built with trusses, congratulate yourself that you have a good roof and that the project will be easier 🙂

We can match another truss, and who knows, we may have even built the truss your looking at. And if it was stick framed, don’t lose hope! We can do this together!

Floor & Wall Height Considerations

Now back to wall and floor heights.

If the heel height is too short and we can’t match it, there are a couple things that can be done. If the ceiling of the new addition can be lowered an inch or 2, this gives us room to build a truss to match on the roof top.

Or if the addition is for a garage, a lot of times the floor is lower than the house floor anyway and it can be adjusted up or down to make a regular framed wall come out slightly lower than the existing house wall.

The other option is to make the walls taller than the existing house and just make the new roof higher than the old with a higher ceiling. Going up and higher than the old roof means you don’t have to match exactly anymore which is also easier. However, this may not be the look you want.

Plan Your Roof in Advance – Give Us A Call!

At any rate, this is a great time to call Timberlake TrussWorks about your project at 580-852-3660! (We service Oklahoma, Kansas, and North Texas)

After reading this article, you will be up on most everything we would tell you! There are a few tricks left in the bag that can be pulled out if for some reason your project doesn’t fall under these general examples.

When you call, we will want to know the heel height and roof pitch and can begin to help you with matching up and getting a design to work. Contacting us at this stage will also give us a chance to stay with you throughout your process. Keeping on top of the details and planning ahead makes it go much smoother for you and us! By planning ahead and knowing your building schedule as well, we can make sure we have the trusses to you on time so when you rip into the existing structure, you don’t have to keep it uncovered for very long.

Matching Existing Truss Worksheet

Want a worksheet to make this easier?

Fill out the form below letting us know you want the worksheet and we’ll get it to you!

This is the complete list of details we need for a perfect match. Some of the details will may not be available until the floor and walls are complete. Filling out as much of this worksheet as possible will only speed the process up and make it go that much quicker and smoother.